Controllable pitch propellers ("CPP") have many advantages over fixed pitch propellers, particularly in vessels that are operated at a variety of speeds and/or loads or that require superior maneuverability. In many commercial applications the increased maneuverability and economical operation can far outweigh the higher initial cost of a CPP. CPPs are widely used in military vessels because of their superior responses to control commands such as speed changes. In some navy ships a quick reversal of the CPP from full speed ahead to full speed astern can stop the ship in less than four ship lengths.
Bird-Johnson Company ("BJCo"), the assignee of the present invention, has for many years manufactured CPP systems of the type having a hub and blade assembly, a main pitch-control hydraulic servo in the hub, an oil distribution box that operates the pitch-control servo, and a control system that controls the oil distribution box. Although the present invention relates to an improved oil distribution box, it is necessary to have a general understanding of the hub and blade assembly, i.e., the CPP.
The BJCo CPP comprises a hub having a plurality of blade-mounting ports, each of which receives a crank ring to which the blade is attached (either integrally or by bolts) and by which the blade is pivoted about an axis substantially perpendicular to the shaft axis to change its pitch setting. Within the hub is a hydraulic servo that consists of a cylinder, a piston reciprocably movable within the cylinder chamber, a piston rod, and a directional valve for controlling the supply and return of hydraulic fluid to and from the cylinder chamber. Each crank ring carries an eccentric crank pin, and the piston rod is affixed to a crosshead that has a slideway slot corresponding to each crank pin. Each slot receives a sliding block having a hole for the crank pin. When the piston rod moves axially, the crosshead moves with it and moves the sliding blocks along arcs centered on the pivot axes of the blades, thereby exerting forces on the crank pins that rotate the crank rings and blades in the mounting ports and changing the pitch setting of the blades. The hub and blade assembly can be controlled to maintain any desired propeller pitch between a maximum ahead pitch and a maximum astern pitch.
As its name implies, the oil distribution ("OD") box of the CPP system distributes oil (the preferred hydraulic fluid) from a pressurized source to the main servo in the hub. To carry out its ultimate function, the OD box is called upon to do several things. First, it provides a rotary coupling or joint between a stationary oil source and feed and return passages through the ship's shafting. Second, it controls the main directional valve of the main servo. Third, it has a built-in emergency pitch positioner that mechanically sets and holds a desired propeller pitch for emergency "take-home" operation.
In the BJCo CPP system the OD box is usually, though not always, mounted on the forward end of the main reduction gear casing by a mounting flange at the aft end of a stationary housing. The aft portion of the OD box housing receives the forward end of a rotating valve rod that extends aftward through the hollow shaft of the output gear of the gear box and all the way aft through the propeller shaft to the directional valve in the propeller hub. The pressure oil supply line is connected to a supply port in the OD box housing. Because the valve rod moves axially within the housing to control the directional valve in the hub, an intermediate oil supply chamber is defined along a length of the valve rod somewhat longer than the stroke of the valve rod by a rotating tube that surrounds, rotates with and is sealed at opposite ends to the valve rod. Pressure oil flows from the supply port into the intermediate supply chamber through journal clearance seals between the stationary OD box housing and the rotating tube. Holes in the valve rod within the supply chamber admit the oil to the valve rod at all positions along its full operating stroke.
Oil returns from the main servo in the propeller hub through the annular passage between the inside wall of the propeller shaft and the outside wall of the valve rod. The tube that forms the intermediate chamber has passages that allow return oil to pass into an oil return section of the chamber within the OD box housing forward of the journal clearance seals. The oil return line of the CPP system hydraulics is coupled to a return port in the housing opening to the oil return section.
Fore and aft movements of the valve rod to control the directional valve in the hub are provided by an auxiliary servo in the OD box. It is useful to mention, at this point, that a further aspect of the BJCo CPP system being described here is the use of the valve rod to provide feedback information, in the form of its axial position, of the pitch setting of the propeller. To this end the directional valve for the main servo is a four-way open center directional control valve, in which the valve body is built into the piston rod and the valve spool is attached to the aft end of the valve rod. With this design the main servo piston tracks (follows) every movement of the valve rod.
Another aspect of the BJCo CPP system design is the capability of mechanical pitch-setting for emergency take-home. When the main servo is inoperative to control the propeller pitch for whatever reason, movement of the valve rod in the forward direction by the auxiliary servo in the OD box engages an abutment on the valve spool against a shoulder on the piston rod, thereby mechanically moving the main servo piston, piston rod and crosshead forwardly to set and maintain mechanically (as far as the hub and blade assembly is concerned) the desired propeller pitch for emergency take-home operation.
Movement of the piston, piston rod, and crosshead mechanically to set and maintain a desired pitch for take-home operation requires the application of a very high pressure to the auxiliary servo cylinder. Moreover, journal clearance seals are inherently controlled leakage seals, which means that maintenance of emergency take-home pitch would require continuous pumping at high pressure to compensate for leakage if the servo cylinder were rotating with the shaft and the oil were supplied through journal clearance seals. Therefore, the OD boxes of the prior designs have a non-rotating auxiliary servo that is coupled to the rotating valve rod by anti-friction thrust bearings. The OD box has a stationary cylinder section adjacent to its forward end that receives two axially spaced-apart pistons coupled to each other by a piston rod and together forming with the cylinder section a working chamber. The working chamber is divided into fore and aft parts by an annular divider wall affixed to the housing and sealed to the piston rod. The pistons, piston rod and wall are non-rotatable and are coupled by an anti-friction thrust bearing to the valve rod. The non-rotating seals of the auxiliary servomotor are fully up to the requirements for maintaining the high oil pressure required for emergency mechanical take-home pitch setting. A mechanical lock is provided at the forward end of the OD box which is engaged with the forward piston by rotating the lock clockwise to a stop. When the mechanical lock is applied, the valve rod is stretched to a pre-loaded state for the purpose of resisting alternating hydrodynamic loads. The preload is transmitted from the valve rod to the non-rotating servo through the thrust bearing, thus limiting the life of the bearing during take-home operation. The reaction of the preload on the servo housing is applied to the forward side of the reduction gear case.
In addition to providing for the establishment of an emergency take-home propeller pitch setting, solely linear motion of the auxiliary servo piston also makes it easy to deliver pitch-setting feedback information by a follow-up rod attached to the piston and extending out through a sealed opening in the forward end wall of the OD box housing.
The above-described BJCo CPP system has been proven by many years of service on dozens of naval vessels throughout the world to be extremely reliable, durable and effective. Like any mechanical device with moving parts periodic overhaul is, of course, necessary. In the case of OD box, for example, the thrust bearings between the valve rod and the auxiliary servo piston have to be replaced periodically. Moreover, as the end of their service life approaches, there is an ever-increasing fear that they might fail if called upon to endure the high loads imposed by running in the emergency take-home mode. In this regard, the load on the thrust bearing when the OD box is in the emergency take-home mode is more than five times as great as that when it is controlling the actuating valve of the main servomotor. In addition the OD boxes of the prior design described above require that the casing of the reduction gear box be designed to carry the reaction load of the valve rod when the system is in the take-home mode, inasmuch as the reaction load is transmitted through the OD box housing to the gear box casing.